Graded spacers in spiral wound elements

ABSTRACT

Embodiments of the present invention provide elements that are beneficial for use in fluid filtration. Embodiments provide elements that have variable feed spacer height, variable permeate spacer height, or both. The variable height allows flow properties to be matched to fluid volume as the filtration occurs.

TECHNICAL FIELD

The subject invention relates to a membrane or filtration systemutilized for the separation of fluid components, specificallyspiral-wound membrane elements and flat membrane elements.

BACKGROUND

Spiral-wound membrane filtration elements generally consist of alaminated structure comprised of a membrane sheet sealed to or around aporous permeate carrier which creates a path for removal, longitudinallyto the axis of the center tube, of the fluid passing through themembrane to a central tube, while this laminated structure is wrappedspirally around the central tube and spaced from itself with a porousfeed spacer to allow axial flow of the fluid through the element.Traditionally, a feed spacer is used to allow flow of the feed water,some portion of which will pass through the membrane, into the spiralwound element and allow reject water to exit the element in a directionparallel to the center tube and axial to the element construction.

An alternate design of a spiral wound element allows for the feed toreject stream to flow in the same longitudinal direction of the spiralwould membrane as the permeate flow, either from the outside of thespiral to the center, or from the inside of the spiral to the outside.Such a design requires a specially modified divided center tube to allowtwo separate flow paths, and also requires that both axial ends of theelement are fully sealed to create the longitudinal flow path. Thisdesign of a spiral would element can be advantageous in providing anincreased flow velocity and fluid shear within the feed to reject streamwhen compared to the traditional cross-flow design. Increased flowvelocity can be beneficial for the prevention of organic and inorganicfouling of the membrane and feed spacer, as well as reducingconcentration polarization within the element as liquid passes throughthe membrane. Examples of such elements are those manufactured byPentair Corporation under the trade name GRO.

It is also a source of flow restriction and pressure drop within theaxial flow channel and also presents areas of restriction of flow andcontact to the membrane that contribute significantly to membranefouling via biological growth, scale formation, and particle capture.

SUMMARY

Embodiments of the present invention provide elements that arebeneficial for use in fluid filtration. Embodiments provide elementsthat have variable feed spacer height, variable permeate spacer height,or both. The variable height allows flow properties to be matched tofluid volume as the filtration occurs.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of a longitudinal flow spiral woundelement.

FIG. 2 is a longitudinal cross section illustration of an unrolledlongitudinal flow element with height graded deposited feed spacer.

FIG. 3 is a longitudinal cross section illustration of an unrolledlongitudinal flow element with height graded deposited feed spacer andheight graded deposited permeate carrier.

FIG. 4 is a longitudinal cross section illustration of an unrolledlongitudinal flow element with deposited fixed height feed spacer and aheight graded mesh permeate carrier.

FIG. 5 is a longitudinal cross section illustration of an unrolledlongitudinal flow element with deposited fixed height feed spacer and aheight graded deposited permeate carrier.

FIG. 6 is a longitudinal cross section illustration of an unrolled axialflow element with deposited fixed height feed spacer and a height gradedmesh permeate carrier.

FIG. 7 is a longitudinal cross section illustration of an unrolled axialflow element with deposited fixed height feed spacer and a height gradeddeposited permeate carrier.

FIG. 8 is a longitudinal cross section illustration of an unrolled axialflow element with deposited height graded feed spacer and a heightgraded deposited permeate carrier.

DESCRIPTION OF EMBODIMENTS AND INDUSTRIAL APPLICABILITY

Improvements to the design of spiral wound elements have been disclosedin U.S. Pat. No. 6,632,357 to Barger et al., U.S. Pat. No. 7,311,831 toBradford et al., and patent application PCT/US14/18813 to Herrington etal. which replaces the feed spacer with islands or protrusions eitherdeposited or embossed directly onto the inside or outside surface of themembrane. None of these patents or applications, however, teaches agraded feed spacer or permeate carrier pattern height perpendicular tothe axis of the center tube. The inventors have discovered that thisconfiguration can be advantageous in that it maintains spacing for axialflow through the element while minimizing obstruction within the flowchannel. In a brine feed spacer, concentration polarization increases atthe reject end of the feed space as water molecules go through themembrane and reject salt ions. By having a thinner feed channel heightat the reject end of the feed space, fluid shear is increased, therebyreducing the negative effects of concentration polarization. A printedor deposited spacer also eliminates the porous brine feed spacer as aseparate component, thus simplifying element manufacture. On thepermeate side of the membrane, as more and more water passes through themembrane, the volume of water flow increases as the water approaches thecenter tube. By gradually increasing the height of the space in thepermeate flow region as fluid approaches the center tube, more volume(or cross section) is available in the space to minimize fluid velocityand subsequently reduce back pressure in the permeate space.

Due to the nature of membrane filtration, the inlet flow volume iscontinuously reduced while it flows across the membrane, and thepermeate flow volume increases down the length of the permeate carrier.The reject flow volume represents the difference between the inlet flowand the permeate flow, and comprises the full content of the dissolvedor suspended materials which are not able to flow through the membrane,and are thus at a higher concentration in the reject stream than theinlet feed stream.

The present invention provides a gradation in the brine feed spacingheight in longitudinal flow spiral wound elements such that the heightof the brine feed spacing decreases from the flow stream to the rejectstream, allowing for more consistent or increased flow velocity as thevolumetric flow through the feed to reject stream is reduced due to flowacross the membrane. FIG. 1 depicts a longitudinal flow spiral woundelement comprising one layer of membrane sheet 1, graded height feedspacer features 2 deposited on a second layer of membrane sheet 9 with afixed height permeate carrier 3. The direction of feed flow and permeateflow are concurrent inward from the outer edge. The height of the feedspacer decreases from the point most distant from the center tube 6 tothe point closest to it. The reject flow and permeate flow are collectedthrough separate openings in a divided center tube 6. Glue lines 7 sealthe edges of the element and produce longitudinal feed to reject flowwithin the element. FIG. 2 shows the same element in an unwoundconfiguration so that the height gradation in the feed spacer features 2can be seen more clearly. In the remainder of the drawings, membranesheet 1 is depicted above and below in the diagram to visually representthe flow paths of the feed and permeate streams, but in use is only asingle membrane sheet which creates the 2^(nd) flow channel when thespiral-wound element is rolled. As the brine flows from the feed endtoward the reject stream at the center tube, volumetric flow in the feedchannel is reduced as water passes through the membrane to the permeatecarrier. Decreasing the height of the feed spacer allows for constant orincreased flow velocity in the feed stream as the volumetric flowdecreases, which can be advantageous in preventing scale formation inthe element. Likewise, by decreasing the height of the feed spacer alongits length, overall leaf length can be increased while keeping theelement diameter constant, allowing for additional surface area withinthe element.

Similarly the height of the permeate carrier can be decreased at thelongitudinal end of the element most distant from the center tube, wherethe volumetric flow is at its minimum, and increased as the volumetricflow increases towards the center tube. The height of the brine feedspacer can be increased at the longitudinal end of the element mostdistant from the center tube while the permeate carrier height isdecreased commensurately so that the overall thickness of the feedspacer, membrane sheet, and permeate carrier are a constant thicknessthroughout the assembly. The heights can also be varied to achieve anincreasing or decreasing overall thickness if desired. The variableheight of both of these spacers can be achieved, as examples, bylayering conventionally utilized mesh spacers of various heights or bydepositing features on the membrane sheet whose height varieslongitudinally directly onto one or both sides of the membrane sheetbefore element assembly. FIG. 3 , depicts an unrolled longitudinal flowelement with a deposited variable height feed spacer 2 which decreasesin height from feed to reject flow between membrane sheets 1, 9. Aheight graded deposited permeate carrier 4 increases in height from theend most distant from the divided center tube 6 to the attachment to thedivided center tube 6; in the example shown the increase in heightmatches the decrease in height of the feed spacer. The permeate carrieris bounded on 3 sides by glue lines 7, while the feed spacer is boundedonly on the outer edges. The direction of feed flow and permeate floware concurrent inward from the outer edge to the divided center tube 6.The combination of decreasing feed height and increasing permeate heightoptimizes the overall volume of the element, maintaining more constantlongitudinal volumetric flow throughout both the feed spacer and thepermeate carrier.

In traditional axial brine feed flow and in longitudinal flow elements,another example embodiment of the present invention employs a gradationin the height of only the permeate carrier, increasing in thicknesslongitudinally from the point most distant from the center tube to thepoint adjacent the center tube. An example industry standard permeatecarrier can be 0.010″ in height throughout its length. By providing apermeate carrier which is thinner, for example 0.002″ at the edge mostdistant from the center tube, increasing in height to 0.010″ at the edgeclosest to the center tube, the length of the envelope comprising themembrane sheet and permeate carrier can be increased, providingadditional surface area for filtration, while maintaining a constantvolume of the element overall. The variable height of this spacer can beachieved, as examples, by layering conventionally utilized mesh spacersof various heights or by depositing features on the membrane sheet whoseheight varies radially directly onto one side of the membrane sheetbefore element assembly. In axial flow elements multiple leaves can beused, with each permeate carrier comprising a graded height spacer.

FIG. 4 shows an embodiment of an unrolled longitudinal flow element withdeposited fixed height feed spacer 5 between membrane sheets 1, 9. Aheight graded mesh permeate carrier 3, created from three separatesheets of thin mesh, increases in height from the end most distant fromthe divided center tube 6 to the attachment to the divided center tube 6bounded, by glue lines 7. The direction of feed flow and permeate floware concurrent inward from the outer edge to the divided center tube 6.In this embodiment, as well as those shown in FIG. 5 -FIG. 7 , thereduced thickness of permeate carrier at distal end enables longer leaflength in a given diameter element due to reduced overall permeatecarrier volume, while maintaining sufficient cross sectional area foradequate permeate flow throughout.

FIG. 5 depicts a further embodiment of an unrolled longitudinal flowelement with deposited fixed height feed spacer 5 between membranesheets 1, 9. Here, a height graded deposited permeate carrier 4increases in height from the end most distant from the divided centertube 6 to the attachment to the divided center tube 6, bounded by gluelines 7. The direction of feed flow and permeate flow are concurrentinward from the outer edge to the divided center tube 6.

In an example embodiment shown in FIG. 6 , an axial flow element employsa deposited fixed height feed spacer 5 between membrane sheets 1, 9 witha height graded mesh permeate carrier 3, created from three separatesheets of thin mesh, which increases in height from the end most distantfrom the center tube to the attachment to the center tube 8, bounded byglue lines 7. Permeate flows longitudinally inward towards the centertube while feed-to-reject flows axially, parallel to the axis of centertube 8.

In another example axial flow embodiment, FIG. 7 shows a longitudinalcross section illustration of an unrolled axial flow element withdeposited fixed height feed spacer 5 between membrane sheets 1, 9 with aheight graded deposited permeate carrier 4 which increases in heightfrom the end most distant from the center tube to the attachment to thecenter tube 8, bounded by glue lines 7. Permeate flows longitudinallyinward towards the center tube while feed-to-reject flows axially,parallel to the axis of center tube 8.

In another embodiment, the height of the brine feed spacer is decreasedfrom the brine feed inlet to its outlet in an axial flow element. Thisconfiguration allows for more consistent or increased brine flowvelocity as the volumetric flow through the feed to reject stream isreduced due to flow across the membrane. Maintaining more constant axialvolumetric flow throughout the feed space can be advantageous inpreventing scale formation in an axial flow element. In this case, theheight of the permeate carrier must be decreased commensurately so thatthe overall thickness of the feed spacer, membrane sheet, and permeatecarrier are a constant thickness throughout the assembly to allow foruniform rolling. The variable height of both of these spacers can beachieved, as examples, by layering conventionally utilized mesh spacersof various heights or by depositing features on the membrane sheet whoseheight varies spirally directly onto one or both sides of the membranesheet before element assembly. FIG. 8 depicts such an embodiment with asection of an axial flow element with deposited variable height feedspacer 2 which decreases in height from feed to reject flow betweenmembrane sheets 1, 9 with a height graded deposited permeate carrier 4which increases in height in the direction from feed to reject flow,bounded by glue lines 7.

Some permeable membrane systems use flat sheets instead of spiral-woundconfigurations and it can be seen that using graded height feed spacers,decreasing in height from inlet to outlet, in conjunction with inverselygraded permeate spacers, can optimize volumetric flow in such systems aswell. The example embodiments described, left in their unrolled statesas illustrated in FIG. 2 -FIG. 8 , illustrate use in flat sheet systems.

Those skilled in the art will recognize that the present invention canbe manifested in a variety of forms other than the specific exampleembodiments described and contemplated herein. Accordingly, departuresin form and detail can be made without departing from the scope andspirit of the present invention as described in the appended claims.

We claim:
 1. An element for use in fluid filtration, comprising: (a) apermeate spacer element, permeable to a fluid; (b) a membrane sheetdisposed next to a first surface of the permeate spacer element; (c) apermeable sheet, permeable to a fluid, disposed next to a secondsurface, opposite the first surface, of the permeate spacer element; and(d) a feed-reject spacer element mounted on a surface of the membranesheet and separated from the permeate spacer element by the membranesheet, or mounted on the permeable sheet and separated from the permeatespacer element by the permeable sheet; (e) wherein, the permeate spacerelement, permeable sheet, membrane sheet, and feed-reject spacer elementare rolled around a central collection tube into a spiral wound element,and the permeate spacer element provides a permeate volume between thepermeable sheet and the membrane sheet, and the feed-reject spacerelement provides a feed-reject volume between the permeable sheet andthe membrane sheet; (f) wherein the central collection tube defines atube permeate volume in the interior of the tube that is incommunication with the exterior of the tube through one or more permeateopenings; (g) wherein the feed-reject spacer element provides aseparation that increases from a first separation proximal the permeateopenings to a second separation, greater than the first separation,distal from the permeate openings.
 2. The element of claim 1, whereinthe feed-reject spacer element comprises an extruded or woven mesh witha varying thickness.
 3. The element of claim 1, wherein the feed-rejectspacer element comprises two or more layers of extruded mesh, wovenmesh, or porous material, stacked to provide the varying thickness. 4.The element of claim 1, wherein the feed-reject spacer element comprisesfeatures deposited on the surface of the membrane sheet facing away fromthe permeate volume, or the surface of the permeable sheet facing awayfrom the permeate volume, or both, where the features project from thesurface on which they are deposited by a height that decreases from afirst height distal from permeate openings to a second height, lesserthan the first height, proximal to the permeate openings.
 5. The elementof claim 1, wherein the permeate spacer element comprises an extruded orwoven mesh with a varying thickness.
 6. The element of claim 1, whereinthe central collection tube defines a tube reject volume in the interiorof the tube, separated from the tube permeate volume, wherein the tubereject volume is in communication with the exterior of the tube throughone or more reject openings.